Manufacture of screen plate with screen slots obtained by laser beam cutting

ABSTRACT

A method for manufacturing a screen plate with screen slots from a thin plate ( 1 ), which screen slots ( 5, 13 ) are achieved by cutting. The plate is formed by bending to a screen plate of a desired shape. The screen slots ( 5, 13 ) are produced by a laser beam for cutting, which hits the plate from one of its sides, the inlet side. Focus for the beam is situated on a predetermined depth between both sides of the plate. The slot is obtained by at least two cutting operations following each other, in which the direction ( 3, 4 ) of the laser beam forms two different angles with the main plane of the plate. There is obtained a screen slot ( 5, 13 ), the cross section of which increases against the other side of the plate, the outlet side.

The present application is the National Stage under 35 U.S.C. 371 ofPCT/SE98/01617, filed Sep. 11, 1998, which designated the United Statesand was published under PCT Article 21(2) in English.

The present invention relates to a method for manufacturing a screenplate from a thin plane plate with screen slots and a screen elementcomprising one or several screen plates. The screen slots are achievedby metal cutting. After this cutting the plate is formed to a screenplate of a desired shape.

To manufacture a cylindrical screen drum from a plate is described in WO93/04797. According to this publication the starting material is arectangular plate on one side of which a number of parallel grooves havebeen obtained by milling. The grooves have V-shape with a small planearea in their bottom. A number of supporting strips are then weldedacross the grooves. From the rear side of the plate screen slots aremilled in front of the grooves. Finally the plate is bent to a drum withthe V-shaped grooves turned outside. Through the bending the screenslots in the finished drum will be thinner than they were in the platein its plane form.

The screen slots manufactured according to this publication are cut in amechanical way. In order to obtain sufficiently thin slots the edges inthe milling equipment must be sharp, which leads to frequent changeswith accompanying high costs.

In U.S. Pat. No. 5,587,077 there is described another solution of theproblem to obtain slot openings in a plate which is formed to a screendrum or screen plate. According to this publication the openings are cutwith a water jet under high pressure. Firstly one produces a number ofgrooves on the side of the plate which in the produced screen plateshall be found on the inlet side. In a second moment there is obtained anumber of openings or slots in the bottom of the grooves using the waterjet which under high pressure is squirted against the plate. From the USpublication it is seen that the formed opening expands towards theoutlet opening with an angle of 4-8°.

EP 0 287 267 describes the manufacture of cylindrical screen drums wherethe cutting of the screen slots takes place by means of laser beams.This cutting takes place firstly when the plate has been formed into acylinder. The laser beam is directed against the outer side of thecylinder and the formed slot obtains a diminishing cross-section withits broadest part on the outlet side of the cylinder. In the embodimentsshown in the drawings the angle between the walls of the slots isrelatively small.

In the description of U.S. Pat. No. 5,587,077 the drawbacks of lasercutting of screen slots are said to be that the screen slot has only aslight conical form (0-2°), that the problems with burrs on the outletside for the laser beam are important and that the cost for cutting ishigh.

According to the invention it now has surprisingly been found that it ispossible, using laser cutting, to obtain a screen plate with highcapacity for the suspension which is to pass the screen plate. Accordingto the method of the invention the screen slots are formed by means of alaser beam for metal cutting, at which the laser beam hits the platefrom one of its sides, the inlet side, and where the focus for the beamis situated on a predetermined depth between both sides of the plate.The slot is created using at least two cutting operations, in which thedirection of the laser beam forms two different angles against the mainplane of the plate, at which there is obtained a screen slot thecross-section of which increases against the other side of the plate,the outlet side. In the described way screen slots with every desiredform, straight, bent or wave-formed, may be obtained. The two cuttingoperations may be obtained with a single laser nozzle which moves to andfro or by use of two laser nozzles which have been orientated inrelation to each other in a desired way. For reason of simplicity fromhereon only one nozzle is mentioned. The different cutting operationsmay as said above take place with at least two nozzles. Of course eachscreen slot may be cut in three or several cutting operations if so isdesired. In such a case the beam may for example firstly hit the screenplate at a right angle to the same and then in a later moment be slantedin order to cut the sloping walls of the screen slot.

According to the method of the invention the laser nozzle is withadvantage arranged to take at least two positions for the cuttingoperation and the point in which said angles cross each other issituated in an area within or above the plate, where the upper limit ishalf the thickness of a plate above the upper surface of the plate andthe lower limit half the plate thickness below the upper surface of theplate. The cross-section of the screen slot depends on where the beamscross each other. The desired cross-section is determined with knowledgeof the application for which the screen plate shall be used.

According to a preferred embodiment the laser nozzle is arranged in sucha way that the laser beam at both cuttings hits the plate along one andthe same line, at which the obtained cross-section of the screen slot onone of its sides is determined by the laser beam at its first cuttingwhile the other side of the cross-section is determined by the laserbeam during the second cutting. In this case the beams cross each otherat the upper surface of the plate.

According to the method of the invention one may with advantage arrangethe laser nozzle in such a way that it is situated at the same distancefrom a line perpendicular to the plate starting from the slot opening.If the cutting is carried through in this way there is obtained a screenslot which has a center line perpendicular to the plate. If one prefersa screen slot the center line of which forms an acute or a blunt anglewith the main plane of the plate the cutting takes place with laserbeams, where the laser nozzle during both cuttings is situated on thesame side of the line perpendicular to the plate starting from the slotopening.

Of course, a cutting may take place with the laser nozzle arranged suchthat the beam hits the plate perpendicularly to the plate in a firstcutting operation, while the second cutting takes place with a beamwhich forms an acute or blunt angle against the plate. The choice ofdirection for the slots depends on the qualities of the suspension whichis to be treated in the screen.

In order to facilitate the transport through the screen plate the wallsof the screen slot is suitably cut such that the walls form an angle aextending 10°. With advantage the angle a is 10-30°.

In many applications it is suitable that the plate on its inlet side haslong parallel grooves in the bottom of which the screen slots are cut.Such a corrugated surface may alternatively be obtained by weldingstrips of a desired shape on the inlet side of the plate.

The screen plate is with advantage produced by cutting the screen slotsin a plate having a thickness of 2-8 mm. The width of the screen slotson the inlet side is 0,1-0.4 mm.

The screen plate may with advantage be ed into a cylindrical screen drumintended for a pressure screen for fiber suspensions.

The present invention is described further with reference to theaccompanying drawings, in which

FIG. 1 shows a cross-section of a screen slot manufactured according tothe method of the invention,

FIG. 2 shows a cross-section of a screen plate with screen slots,

FIG. 3 shows a cross-section A—A of a screen slot in FIG. 2,

FIG. 4 shows a cylindrical screen with screen plates manufacturedaccording to the invention and

FIG. 5 shows schematically a longitudinal section through a pressurescreen plant in which the cylindrical screen according to FIG. 4 may beused.

The attached drawing FIGS. 1-3 show a method of manufacturing a screenplate chosen as an example.

In FIG. 1 there is shown a cross-section of a part of a thin plate 1provided with parallel transverse grooves 2. With dotted lines there isshown the direction of two laser beams 3 and 4 which create a slot 5 inthe plate. The laser beams 4 and 5 start from a laser nozzle (not shown)arranged above the plate.

In a first cutting operation the laser nozzle is directed against theplate in such a way that a laser beam with a direction 3 forms an anglewith the main plane of the plate (in this example a plane parallel withthe bottom 6 of the plate). The laser beam hits the upper side of theplate in a point 7 and cuts a section through the plate 1. The focus ofthe laser beam is situated on a predetermined depth between both sidesof the plate in order to obtain an optimal cutting. The laser beam isthen moved along the screen plate in order to cut a slot of a desiredlength. When the slot length has been obtained the laser nozzle is movedsuch that the laser beam now will have another direction 4 against themain plane of the plate. The beam will still hit the plate 1 in thepoint 7 as it is returned to the first end of the slot along the sameline in the plate as the laser beam followed during the first cuttingoperation. The formed slot has consequently a cross-section whichincreases from the upper side of the plate to the lower. The walls inthe slot form as may be seen in FIG. 1 an angle a with each other. Inthe embodiment shown on the drawing the angle is 20°. As said earlier ais suitably between 10°and 30°. The slot has in its upper, narrower parta width of 0,2 mm.

In FIG. 2 there is shown a cross-section of a larger part of a screenplate produced according to the method of the invention.

In FIG. 2 reference is made to a cross-section A—A, which may be seen inFIG. 3 and shows the slot in longitudinal section along its length L. Byforcing the laser nozzle to another angle when the beam shall changedirection from 3 to 4 the beam may be forced to cut away a little partof the plate below the end points of the slot, such that the length ofthe slot is larger on the underside of the plate than on its upper side.This operation facilitates the removal of the part of the plate whichshall be taken away.

In FIG. 4 there is shown an example of a shape of a manufactured screendrum with screen plates produced according to the method of theinvention. The screen drum 8 comprises four separate cylindrical screenelements 9-12, which have been formed from four plates provided withscreen slots 13 produced according to the method of the invention. Theplates have been bent such that the inlet side of the plate is situatedin the inner side of the screen drum. The slots consequently increasetowards the outside. The four screen elements are detachably connectedto each other by annular connection means 14. Alternatively the screendrum 8 may be designed in one piece. The screen drum has an inlet end 15and a reject end 16.

In the drawing there is also shown a rotor 17 with rotor wings 18arranged within the screen drum and the direction of rotation of therotor. The long, parallel grooves in the screen plate are numbered 2.

The screen drum 8 which is shown in FIG. 4 may be used in the pressurescreen schematically shown in FIG. 5. Such a pressure screen comprises ahollow house 19, the stationary screen drum 8 which divides the inner ofthe house 19 into a chamber 20 for pulp suspension which is to bescreened through the screen drum 8 and an annular chamber 21 forcollection of screened pulp suspension. The rotor 17 is operated by amotor 22. The rotor is journalled in bearings 23. The rotor 17 has rotorwings 18 extending axially along the rotor 17 at a small distance fromthe screen drum 8.

The rotor wings 18 prevent clogging of the screen slots 13 and maintaina flow of suspension in the circumferential direction of the screen.

The chamber 20 has an inlet 24 for supply of pulp suspension which is tobe separated in the screen drum 8. The inlet end of the screen drum isnumbered 15 and its outlet part with 16. At the outlet part 16 there isa reject outlet 25 with a valve 26 for control of the reject flow. Theannular chamber 21 has an accept outlet 27 for purified suspension.

What is claimed is:
 1. A method for manufacturing a screen plate withscreen slots from a thin plate (1), which screen slots (5, 13) areobtained by cutting, after which the plate is bent into a screen plateof a desired shape, characterized in that the screen slots (5,13) areobtained by a laser beam for cutting, at which the laser beam hits theplate from one side, the inlet side, and that focus for the beam issituated on a predetermined depth between operations following eachother, in which the direction (3,4) of the laser beams has two differentangles against the main plane of the plate, at which there is obtained ascreen slot (5, 13), the cross-section of which increases against theother side of the plate, the outlet side.
 2. A method according to claim1, characterized in that the laser nozzle is arranged to take at leasttwo positions for the cutting operation and that the point in which saidangles (3, 4) cross each other is situated in an area in or above theplate, where the upper limit is half the plate thickness above the uppersurface of the plate and the lower limit is half the plate thicknessbelow the upper surface of the plate.
 3. A method according to claim 2,characterized in that the laser nozzle is arranged in such a way thatthe laser beam during said at least two cuttings meets the plate alongone and same line, at which the cross-section of the obtained screenslot on one of its sides is determined by the laser beam at its firstcutting, while the other side of the cross-section is determined by thelaser beam at the second cutting.
 4. A method according to claim 3,characterized in that the laser nozzle during said at least two cuttingoperations is situated on the same distance from a line perpendicular tothe plate starting from the slot opening, at which a screen slot (5, 13)is obtained which has a center line perpendicular to the plate.
 5. Amethod according to claim 3, characterized in that the walls of thescreen slot (5, 13) form an angle ac exceeding 10°.
 6. A methodaccording to claim 5, characterized in that the laser nozzle is arrangedto make a turning movement at the end points of the slot in such a waythat the laser beam forms a slot with an enlarged cross-section alsoseen in its longitudinal section.
 7. The method of claim 5 wherein saidangle α is 10-30°.
 8. A method according to claim 1, characterized inthat the plate has a thickness of 2-8 mm and that the screen slots havea width of 0,1-0,4 mm on the inlet side.
 9. A method according to claim1, characterized in that the screen plate is formed to a cylindricalscreen drum intended for a pressure screen for fibre suspensions.
 10. Amethod according to claim 1, characterized in that the plate on theinlet side has long parallel grooves (2), in the bottom of which thescreen slots are cut.